Atrial synchronized dual chamber pacing modes, particularly, the multi-programmable, VDD, VDDR, DDD and DDDR pacing modes, have been widely adopted in implantable dual chamber pacemakers for providing atrial and ventricular or AV synchronized pacing on demand. A pacemaker implantable pulse generator (IPG) capable of pacing in atrial synchronized modes typically includes an atrial sense amplifier to detect atrial depolarizations or P-waves and generate an atrial sense event (A-SENSE) signal, a ventricular sense amplifier to detect ventricular depolarizations or R-waves and generate a ventricular sense event (V-SENSE) signal, a ventricular pacing pulse generator and for DDD or DDDR mode pacing an atrial pacing pulse generators as well, providing ventricular and atrial pacing (V-PACE and A-PACE) pulses, respectively, and an operating system governing pacing and sensing functions. The IPG supplies a V-PACE pulse to the ventricles through an appropriate lead system if the ventricles fail to depolarize on their own during an AV delay timed from a preceding A-SENSE or generation of an A-PACE pulse. In DDD or DDDR mode pacing If the atria fail to spontaneously beat within a pre-defined time interval (atrial escape interval), the pacemaker also supplies an A-PACE pulse to the atria through an appropriate lead system. Such AV synchronous pacemakers which perform this function have the capability of tracking the patient's natural sinus rhythm and preserving the hemodynamic contribution of the atrial contraction over a wide range of heart rates. Maintenance of AV mechanical synchrony is of great importance as set forth in greater detail in commonly assigned U. S. Pat. No. 5,626,623, incorporated herein by reference in its entirety.
Typically, the IPG comprises a microcomputer controlled, digital controller/timer circuit that defines and times out a V-A interval (in DDD and DDDR modes) or a V-V interval (in VDD and VDDR modes) upon a V-SENSE or V-PACE pulse and times out an AV delay in response to an A-SENSE (in VDD, VDDR, DDD, DDDR modes) or in response to an A-PACE pulse (in DDD and DDDR modes) as well as a number of other intervals. In some DDD and DDDR mode pacers, separate AV delays are commenced by the A-SENSE signal (an SAV delay) and the A-PACE pulse (a PAV delay).
Additional intervals timed by the IPG include atrial and ventricular sense amplifier blanking periods following delivery of a atrial and/or ventricular pacing pulses to disable atrial and ventricular amplifier sensing. In addition, a number of sense amplifier refractory periods are timed out on atrial and ventricular sense event signals and generation of A-PACE and V-PACE pulses, whereby "refractory" A-SENSE and V-SENSE signals during such refractory periods are selectively ignored or employed in a variety of ways to reset or extend time periods being timed out. Atrial and ventricular refractory periods (ARP and VRP) are commenced upon an A-SENSE or V-SENSE signal or generation of an A-PACE or V-PACE pulse, respectively. The ARP extends through the SAV delay or the PAV delay until a certain time following a V-SENSE signal terminating the SAV or PAV delay or generation of a V-PACE pulse at the expiration of the SAV or PAV delay.
In addition, a post-ventricular atrial refractory period (PVARP) is commenced by a V-PACE pulse or V-SENSE based on the understanding that A-SENSE signals sensed during its time-out generally reflect a retrograde conduction of the evoked or spontaneous ventricular depolarization wave and therefore are not employed to reset an escape interval and commence an SAV delay. The duration of PVARP may be fixed or vary as a function of sensed atrial rate or pacemaker defined pacing rate, with the result that in many cases relatively long PVARPs are in effect at lower rates.
The rate-adaptive VDDR and DDDR pacing modes function in the above-described manner but additionally provide rate modulation of a pacing escape interval between a programmable lower rate and an upper rate limit (URL) as a function of a physiologic signal or rate control parameter (RCP) related to the need for cardiac output developed by a physiologic sensor. At times when the intrinsic atrial rate is inappropriately high or low, a variety of "mode switching" schemes for effecting switching between tracking modes and non-tracking modes (and a variety of transitional modes) based on the relationship between the atrial rate and the sensor derived pacing rate have been proposed as exemplified by commonly assigned U.S. Pat. No. 5,144,949, incorporated herein by reference in its entirety.
The disruption of AV electrical and mechanical synchrony frequently arises due to the spontaneous depolarization of the ventricles triggered at an ectopic site in one of the ventricles. Such a spontaneous depolarization that is not associated with a prior atrial depolarization is characterized as a premature ventricular contraction (PVC). Many of the problems resulting from the occurrence of a PVC in a patient with a dual chamber pacemaker are described more fully in U.S. Pat. Nos. 4,788,980 and 5,097,832, both of which are incorporated herein by reference. One such problem is the initiation of pacemaker mediated tachycardias or PMTs. The most commonly employed PVC response to prevent initiation of PMTs is to extend the PVARP to a programmed duration, such as 400-500 msec., in response to the PVC, thus masking atrial sense signals that are presumed to result from retrograde conduction during this period of time as disclosed in the above-incorporated '980 patent. Numerous other patents have dealt with varying the PVARP in an attempt to prevent instigating a PMT, including U.S. Pat. Nos. 4,920,965, 4,554,921, 5,123,412, and 4,503,857, all incorporated herein by reference in their entireties.
Unfortunately, in some circumstances prolongation of the PVARP in response to a PVC has unfortunate consequences. Even though a possible PMT is prevented, loss of normal P-wave tracking may occur because the next P-wave is masked by the PVC response. Subsequent R-waves occurring prior to the time of the next scheduled atrial pacing pulse or non-refractory sensed atrial depolarization in DDD and DDDR modes or prior to the next scheduled ventricular pacing pulse or non-refractory sensed atrial depolarization in VDD or VDDR modes initiate a subsequent PVARP. If the subsequent PVARP is long enough, the next P-wave may fall therein and fail to initiate an A-V delay. Loss of atrial synchrony may extend over a period of seconds to hours depending on the pacemaker's programmed rate settings and the patient's sinus rate (i.e., the P-wave rate set by the SA node). Ventricular pacing remains inhibited until either the occurrence of a non-refractory sensed atrial depolarization (VDD, VDDR, DDD, or DDDR mode pacers), delivery of an atrial pacing pulse (ODD or DDDR mode pacers) or delivery of a ventricular pacing pulse on lower rate time-out (VDD or VDDR mode pacers).
A similar problem may arise in response to other events which disrupt AV synchrony. Additional events which disrupt AV synchrony can include premature atrial contractions, noise sensing and associated asynchronous pacing, also io known as "noise reversion" and other pacing mode or operation changes, including those arising from mode switching, magnet application and removal, cancel magnet commands, device programming and downlink telemetry functions. In particular, changes from non-atrial synchronized pacing modes to atrial synchronized pacing modes or from non-atrial synchronized operation to atrial synchronized operation within an atrial synchronized pacing mode have the potential to disrupt AV synchrony. Prolongation of PVARP in response to such other disrupting events is disclosed in U.S. Pat. No. 4,554,920, also incorporated herein by reference in its entirety.
In modern dual chamber pacemakers, the current PVARP may vary as a function of programming and/or vary between minimum and maximum PVARPs as a function of current pacing rate or sensed atrial rate. In the context of pacemakers which employ PVARPs which vary as a function of pacing rate or sensed atrial rate, the relatively long PVARPs which may be in effect at lower rates can in the same fashion result in persistent loss of AV synchrony due to PVCs or other disrupting events even in the absence of extension of the PVARP.
What is needed, therefore, is a system for avoiding the persistent loss of AV synchrony resulting from temporary disruption of AV synchrony while retaining the ability to avoid pacemaker mediated tachycardias.